As is well known according to the F-T synthesis method developed by Fischer and Tropsch, who were chemists in Germany in 1923, it is now possible to produce liquid hydrocarbons from synthesis gas derived from coal, natural gas, biomass and the like. The process to produce the liquid hydrocarbons from coal is called a CTL (Coal-to-liquids, referred also to as a coal liquefaction technology) process; the process to produce the liquid hydrocarbons from the natural gas is called a GTL (Gas-to-liquids, referred also to as a natural gas liquefaction technology) process; and the process to produce the liquid hydrocarbons from biomass is called a BTL (Biomass-to-liquid, referred also to as a biomass liquefaction technology) process. In recent years, all similar processes are commonly called XTL technology.
These processes first convert raw materials (e.g., coal, natural gas and biomass) into synthesis gas using a method of gasification, reforming, or the like. The composition of the synthesis gas suitable for the XTL process to produce a liquid fuel preferably uses the ratio of hydrogen to carbon monoxide which becomes about 2 as expressed by the following equation.CO+2H2+—[CH2]−n→—[CH2]−n+1+H2O
where CO, H2, —[CH2]−n, and H2O are carbon monoxide, hydrocarbons, hydrocarbon with a chain length n (the number of carbons, n), and water, respectively. However, as the proportion of hydrogen increases, the selectivity of methane becomes higher and the selectivity of C5+ (hydrocarbons with n≧5) is relatively reduced, so this method is not suitable. Further, a by-product is also produced, such as olefin and oxygenate (molecule containing oxygen atoms such as alcohol, aldehyde, ketone, etc.), as well as the hydrocarbons in the form of paraffin having a linear chain as described above.
Since one of the main goals of the XTL process is to obtain the liquid fuel, a recent trend aims to optimize a cobalt-base catalyst, ratio of hydrogen to carbon monoxide, temperature, and pressure of the synthesis gas, and others to yield linear hydrocarbons, in particular, linear hydrocarbons of C5+ with high selectivity.
Except for the cobalt-based catalyst, an iron-based catalyst is also widely used as a catalyst. The iron-based catalyst, which has been mainly used at an early stage, is less expensive than the cobalt-based catalyst and has low methane selectivity at high temperature and higher olefin selectivity among hydrocarbons. Further, the iron-based catalyst is used to produce olefin-based products, in addition to the liquid fuel.
In contrast, the cobalt-based catalyst is mainly used to produce the liquid fuel while producing less carbon dioxide and has a relatively long lifespan. However, the cobalt-based catalyst is extremely expensive in comparison to the iron-based catalyst, and its methane selectivity increases at high temperature, which requires a reaction at a relatively low temperature. Further, since the cobalt-based catalyst is expensive, it is necessary to distribute it well and use a small amount on the surface of a support. A compound such as alumina, silica, titania, etc. may be used as the support, and a noble metal such as Ru, Pt, Re, and the like may be used as a promoter to improve the performance of the cobalt-based catalyst.
Several types of reactors have been studied to date such as a tubular fixed bed reactor, a fluidized bed reactor, a slurry phase reactor, a micro-channel reactor or multi-channel reactor with a heat exchanger, and the like. A representative fluidized bed reactor may include a circulating fluidized bed reactor and a fixed fluidized bed reactor. Since reaction characteristics and distribution of products vary depending on the shape of the reactor and the reaction condition, it is necessary to select a catalyst appropriately depending on the final product of interest.
In the existing commercialization process more than 10,000 BPD, the fluidized-bed reactor (available from SASOL Limited) and a tubular fixed bed reactor (available from Royal Dutch Shell plc.) have been mainly used.
However, these reactors are suitable for relatively large-scale gas fields. Therefore, a need exists for a more compact and highly efficient reactor suitable for gas fields that are much smaller, or the use of the wasted associated gas.
In recent years, as considerable attention has been paid to a FPSO (Floating Production, Storage and Offloading) process which is designed to produce while searching for resources and loading and unloading at a place where there is a demand, a study on the process having a small scale but high efficiency has been promoted globally. GTL (Gas-To-Liquids) FPSO is a GTL plant on ships having a limited space, and thus it is beneficial that volume of the reactor relative to production is as small as possible. Therefore, it is believed that the multi-channel reactor or the micro-channel reactor among the reactors as described above is the most promising type of reactor.
The micro-channel reactor is fabricated in a structure in which a catalytic reaction unit and a heat exchange unit are alternately stacked, wherein any one of them is composed of micro-channels. When the heat exchange unit is configured with the micro-channel, the catalytic reaction unit may be configured with a fixed layer of a slab type or the catalytic reaction unit may also be configured with the micro-channels. In the catalytic reaction unit composed of the micro-channels, the micro-channels may be filled with the catalyst by inserting it therein or the catalyst may be attached to the inner wall of the reactor using a coating method.
Such FT reactors are particularly suitable for producing diesel, lube base oil and waxes and are operated mainly in a low temperature F-T process.
During a low temperature F-T process, a hydrocarbon with a high boiling point more than diesel is produced over 60%. Therefore, the diesel is additionally manufactured through subsequent steps such as a hydrocracking process and the like, and wax ingredient is converted into high quality lube base oil through a dewaxing process.
The tubular fixed bed reactor and the slurry phase reactor that are representative of the low-temperature F-T reaction have several advantages, but also have a great disadvantage in size compared to the micro-channel reactor or the multi-channel reactor.
The tubular fixed bed reactor has advantages, such as a burden for scaling-up is relatively low, and a mechanical loss of the catalyst is small. Despite the merits, this type of reactor requires an enormous volume relative to production capacity, and the cost for installation and construction is known to be expensive. In addition, since it has a relatively low heat and mass transfer efficiency inside the catalyst layer, it is hard to control the highly exothermic or highly endothermic reaction.
The slurry phase reactor is less expensive in terms of construction costs and equipment costs, and it also has a relatively high heat and mass transfer efficiency. However, in order to scale-up this type of reactor, the complex hydrodynamic behavior inside the reactor should be rigorously analyzed, which makes the design very difficult. In addition, this type of reactor usually suffers from a mechanical loss of catalyst particles due to the collision and friction.
The multi-channel reactor (hereinafter, referred to inclusive of the micro-channel reactor) is a reactor having maximized heat transfer efficiency so that the reaction can occur at high space velocity. The multi-channel reactor occupies less volume relative to the production capacity (about ⅕ to ½ the level relative to a conventional reactor), and its construction and equipment cost is relatively low. Further, it could be scaled-up by numbering-up. Due to the absence of collision and friction of catalyst particles in the bed, the mechanical loss of catalyst particles could be significantly reduced. In addition, even in the case of movement of the reactor, the change of reactor outcome could be minimized and the mechanical loss of catalyst is expected to be negligible.
However, in the case where the catalyst is wash-coated on the wall of the reactor such as a wall reactor, it is extremely hard or nearly impossible to replace the catalyst when the catalyst's life has ended. In a type of fixed-bed, the replacement of the catalyst is relatively easy, but the heat transfer efficiency decreases compared to the type of a wall-coated reactor that is wash-coated on the wall thereof.